Buggy code:
public class StrBuilder implements Appendable , CharSequence { static final int CAPACITY = 32 ; private static final long serialVersionUID = 7628716375283629643L ; protected char [ ] buffer ; protected int size ; private String newLine ; private String nullText ; public StrBuilder ( ) { } public StrBuilder ( int initialCapacity ) { } public StrBuilder ( String str ) { } public String getNewLineText ( ) { } public StrBuilder setNewLineText ( String newLine ) { } public String getNullText ( ) { } public StrBuilder setNullText ( String nullText ) { } public int length ( ) { } public StrBuilder setLength ( int length ) { } public int capacity ( ) { } public StrBuilder ensureCapacity ( int capacity ) { if ( capacity > ( buffer . length ) ) { char [ ] old = buffer ; <START_BUG> buffer = new char [ capacity ] ; <END_BUG> System . arraycopy ( old , 0 , buffer , 0 , size ) ; } return this ; } public StrBuilder minimizeCapacity ( ) { } public int size ( ) { } public boolean isEmpty ( ) { } public StrBuilder clear ( ) { } public char charAt ( int index ) { } public StrBuilder setCharAt ( int index , char ch ) { } public StrBuilder deleteCharAt ( int index ) { } public char [ ] toCharArray ( ) { } public char [ ] toCharArray ( int startIndex , int endIndex ) { } public char [ ] getChars ( char [ ] destination ) { } public void getChars ( int startIndex , int endIndex , char [ ] destination , int destinationIndex ) { } public StrBuilder appendNewLine ( ) { } public StrBuilder appendNull ( ) { } public StrBuilder append ( Object obj ) { } public StrBuilder append ( CharSequence seq ) { } public StrBuilder append ( CharSequence seq , int startIndex , int length ) { } public StrBuilder append ( String str ) { } public StrBuilder append ( String str , int startIndex , int length ) { } public StrBuilder append ( StringBuffer str ) { } public StrBuilder append ( StringBuffer str , int startIndex , int length ) { } public StrBuilder append ( StrBuilder str ) { } public StrBuilder append ( StrBuilder str , int startIndex , int length ) { } public StrBuilder append ( char [ ] chars ) { } public StrBuilder append ( char [ ] chars , int startIndex , int length ) { } public StrBuilder append ( boolean value ) { } public StrBuilder append ( char ch ) { } public StrBuilder append ( int value ) { } public StrBuilder append ( long value ) { } public StrBuilder append ( float value ) { } public StrBuilder append ( double value ) { } public StrBuilder appendln ( Object obj ) { } public StrBuilder appendln ( String str ) { } public StrBuilder appendln ( String str , int startIndex , int length ) { } public StrBuilder appendln ( StringBuffer str ) { } public StrBuilder appendln ( StringBuffer str , int startIndex , int length ) { } public StrBuilder appendln ( StrBuilder str ) { } public StrBuilder appendln ( StrBuilder str , int startIndex , int length ) { } public StrBuilder appendln ( char [ ] chars ) { } public StrBuilder appendln ( char [ ] chars , int startIndex , int length ) { } public StrBuilder appendln ( boolean value ) { } public StrBuilder appendln ( char ch ) { } public StrBuilder appendln ( int value ) { } public StrBuilder appendln ( long value ) { } public StrBuilder appendln ( float value ) { } public StrBuilder appendln ( double value ) { } public StrBuilder appendAll ( Object [ ] array ) { } public StrBuilder appendAll ( Collection < ? > coll ) { } public StrBuilder appendAll ( Iterator < ? > it ) { } public StrBuilder appendWithSeparators ( Object [ ] array , String separator ) { } public StrBuilder appendWithSeparators ( Collection < ? > coll , String separator ) { } public StrBuilder appendWithSeparators ( Iterator < ? > it , String separator ) { } public StrBuilder appendSeparator ( String separator ) { } public StrBuilder appendSeparator ( char separator ) { } public StrBuilder appendSeparator ( String separator , int loopIndex ) { } public StrBuilder appendSeparator ( char separator , int loopIndex ) { } public StrBuilder appendPadding ( int length , char padChar ) { } public StrBuilder appendFixedWidthPadLeft ( Object obj , int width , char padChar ) { } public StrBuilder appendFixedWidthPadLeft ( int value , int width , char padChar ) { } public StrBuilder appendFixedWidthPadRight ( Object obj , int width , char padChar ) { } public StrBuilder appendFixedWidthPadRight ( int value , int width , char padChar ) { } public StrBuilder insert ( int index , Object obj ) { } @ SuppressWarnings ( "null" ) public StrBuilder insert ( int index , String str ) { } public StrBuilder insert ( int index , char [ ] chars ) { } public StrBuilder insert ( int index , char [ ] chars , int offset , int length ) { } public StrBuilder insert ( int index , boolean value ) { } public StrBuilder insert ( int index , char value ) { } public StrBuilder insert ( int index , int value ) { } public StrBuilder insert ( int index , long value ) { } public StrBuilder insert ( int index , float value ) { } public StrBuilder insert ( int index , double value ) { } private void deleteImpl ( int startIndex , int endIndex , int len ) { }

Fix:
buffer = new char [ capacity * 2 ] ;